ceph: make ceph_fill_trace and ceph_get_name decrypt names

When we get a dentry in a trace, decrypt the name so we can properly
instantiate the dentry or fill out ceph_get_name() buffer.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Reviewed-and-tested-by: Luís Henriques <lhenriques@suse.de>
Reviewed-by: Milind Changire <mchangir@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>